This invention relates to drilling and cementing boreholes.
The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole.
Once the borehole has been drilled to the desired depth, it may be desirable to isolate the separate areas, zones or formations transversed by the borehole. For extraction of fluids from formations, a conduit (casing) must be inserted into the borehole extending from the surface downward, and liners may be hung inside the casing.
At this point it becomes necessary to fill the annulus between the casing and the borehole wall or between the liner and casing with a material which will seal the annulus and provide structural support for the casing or liner. This is commonly referred to as primary cementing.
Generally, the area below and surrounding the casing or liner being cemented is filled with drilling mud. Therein lies a problem that has created primary cementing difficulties throughout the history of cementing. Conventional Portland cement and conventional drilling muds are incompatible. Thus, as the cement is forced down the casing or liner and up into the annulus, mixes with drilling mud and the area around the end of the casing or liner being cemented as well as varying lengths of the annulus end up being filled with a mixture which does not set up into a strong cement.
It is generally necessary after setting each casing or liner to test the integrity thereof by means of a pressure test. However, if the area at the bottom end thereof is not sealed off, pressure testing fails. Hence, the industry has developed a complex system to circumvent this problem. A device generally known as a landing collar is attached to the bottom of the casing or liner being cemented. The casing or liner has an annular shoulder projecting inwardly. The cementitious slurry is introduced into the casing followed by a rubber wiper plug. Displacement fluid then pushes the wiper plug downward, thus displacing the cementitious slurry out of the casing and into an area at the bottom thereof where it changes direction and flows upwardly into the annulus. When the wiper plug reaches the area of the landing collar, it is stopped by the inwardly projecting shoulder and forms a seal so as to allow pressure testing, although a good seal is not always obtained. These problems are magnified with large diameter casings and liners.
Slag, broadly, as a cementitious material is shown in Tragessar, U.S. Pat. No. 3,557,876 (Jan. 26, 1971).
Cementing of the annulus between an oil well casing and the surrounding borehole has been practiced for over 90 years. Long ago, it was recognized that it would be advantageous to solidify drilling fluid in the annulus so as to reduce the cost of the cementing of the casings. Over the decades, various attempts have been made along this line as evidenced by the disclosures in Williams et al., U.S. Pat. No. 2,649,160 (Aug. 18, 1953), and Miller et al, U.S. Pat. No. 3,887,009 (Jun. 3, 1975). However, such techniques, while presumably operable, have failed to achieve commercialization even after the passage of many years. Another attempt in this direction is described in Wyant et al., U.S. Pat. No. 3,499,491 (Mar. 10, 1970), wherein Portland cement is used in an aqueous drilling fluid to produce a mud concrete. Portland cement, however, is very sensitive to the water/solids ratio. Even under ideal conditions, only small increases in the amount of solids results in a very viscous mass. On the other hand, only a slight decrease in the amount of solids results in a composition which sets up to give a very weak structure. These problems are amplified when attempting to use Portland cement in a drilling mud. Thus, while this technique has been used commercially, it has serious drawbacks.
Clarke, U.S. Pat. No. 4,761,183 (Aug. 22, 1988) and Clarke, U.S. Pat. No. 5,106,423 (Apr. 21, 1992) disclose finely ground slag in cementitious slurries for grout. The '423 patent, at column 2, line 6, refers to petroleum utilities.